Changed Canvas::handle() so that CTRL Left Click has drag threshold and toggles note selection if not dragged.
Fixed SHIFT range selection.
ALT Left Click drag is now exclusively vertical or horizontal (better than unintentionally changing the other parameter).
Redo support
Redo grouping of move, duration and velocity change operations.
Now requiring GCC with C++11 functionality for atomic operations (GCC 4.3 or later).

Grid changes:
Renamed _rd and _rw to _ro_data and _rw_data.
Changed _ro_data to atomic pointer type.
Added _in_rt_thread atomic boolean to track when it is safe to free event history.
Fixed race condition with memory management of read only event data and RT thread.
Renamed _history to _undo_history and added _redo_history.
Added Grid::set_undo_group and reset_undo_group for grouping operations in undo state.
Merge branch 'master' of https://github.com/Element-Green/non